1. Field of the Invention
The present invention relates to an apparatus and method for detecting a null symbol. More particularly, the present invention relates to an apparatus and method for detecting a null symbol in an input signal of, for example, a digital audio broadcasting system.
2. Description of the Related Art
Referring to FIG. 1, a digital audio broadcasting (DAB) signal includes a plurality of frames. As shown in FIG. 2, each of the frames includes two types of reference symbols and a plurality of data symbols. The first type of reference symbol is a null symbol that is used in frame synchronization, has a strength of zero, and is typically positioned at a frontmost end of a frame. The second type of reference symbol is a phase reference symbol.
Referring to Table 1 below, a DAB system supports four different transmission modes. A length of the null symbol in each frame is different in these modes.
TABLE 1Mode 1Mode 2Mode 3Mode 4Null symbol Length1.297 ms324 μs168 μs648 μs
A receiving end in the DAB system must determine a starting point of all the frames in a received signal (that is, a starting point of a null symbol), and must further determine which of the four modes the signal is in.
U.S. Pat. No. 7,006,577 discloses an apparatus for detecting a transmission mode using null symbol length which is determined by detecting the starting and end points of a null symbol. This conventional apparatus includes a first window buffer and a second window buffer that store square values of samples for the first and the second search periods, respectively. However, since the apparatus of U.S. Pat. No. 7,006,577 requires two window buffers, the apparatus occupies significant system resources and cannot perform real-time processing.
U.S. Pat. No. 6,731,702 and U.S. Pat. No. 7,065,171 disclose related conventional methods. However, these two methods are unable to determine the transmission mode of signals, and a receiving end must know in advance the transmission mode of the signals, and the lengths of a null symbol and a frame in order to determine a starting point of a null symbol. Therefore, these methods are impractical in actual use.
Another important consideration in DAB systems is that related to fading of a received signal. In particular, if a receiving end in a DAB system is installed on a vehicle, a fading phenomenon may occur in a channel between a transmission end and the receiving end, particularly when the vehicle is traveling at a high speed. Therefore, when detecting a null symbol, this phenomenon must be taken into consideration so as to prevent the erroneous detection of null symbols.